The present invention is directed to an apparatus for use in turning steerable vehicle wheels.
In a known power steering system, an engine driven pump provides a fixed volume of fluid output per revolution during operation of the pump. Therefore, the rate of flow of fluid from the engine driven pump is proportional to engine speed. The pump in this known power steering system is sized to provide an acceptable rate of fluid flow when the engine is idling and thus provides higher rates of flow at higher engine speeds.
A known power steering motor assembly has an open center valve, which requires a constant rate of fluid flow independently of engine speed. In order to obtain a constant rate of fluid flow independently of engine speed, a known power steering system utilizes a flow control valve having a spring loaded valve spool, which is moved to maintain a constant pressure drop across a control orifice.